The invention relates to a method and a device for the construction of tunnels according to the precharacterizing parts of the independent claims. They are used when a tunnel is to be driven through soil with parameters expected to have a limited life-time. The word "tunnel", in this connection, is to be understood in the general sense of the word. It relates to any kind of tubes that are to be driven into the ground, for instance to more or less horizontally extending street tunnels or canals, but also to underground chambers and cavities.
In order to be able to construct tunnels in the above situation it must be prevented that a just finished round collapses before a support has been installed or that loose pieces of rock fall into the tunnel from the tunnel wall. Known methods in this connection are the shotcrete construction method and mechanical tunnel driving with and without a shield structure.
In the shotcrete construction the tunnel is driven by means of excavators or sectional cutting-machines. Deformations of the tunnel tube immediately after driving a round are allowed so that shape changing resistances in the form of a supporting ring become effective. This supporting ring surrounds the cavity and prevents the ground from intruding into the cavity any further. However, the deformation must not become so considerable that this results in a breaking up due to overload. A thin shotcrete protection limits this deformation by providing an increasing spring-like resistance to the deformation as the latter increases. The main field of application for shotcrete constructions is rock material. The latter may be slightly or heavily jointed, or may have worked loose. Cohesive and noncohesive unstable rock formations are possible fields of application.
In the case of the mechanically driven shield tunnel the exploiting system works with mechanical tools, the tools--if provided as a full cutting-machine with a cutting-wheel or a prospecting wheel--being able to process the entire excavation surface simultaneously. If using them in the form of a sectional cutting-machine the working face is removed in several attacks. The shield structure is a support that wanders along with the tunnel machine, under the protection of which the ground support is installed. Tunnel machines including a shield structure and a cutting-wheel are used in loose rock with an unsupported working face, whilst the machines including a prospecting-wheel are employed in the case of a supported working face. The sectional cutting-machine is used for an unsupported working face.
Exploitation at the working face in the case of a mechanically driven tunnel without a shield structure is the same as that with the mechanically driven tunnel including a shield structure. During its use the machine is anchored in the surrounding ground. The supporting work is done at a later time, separately of the advancing work. The field of use of this machine is rock material.
The construction method with shotcrete has the following shortcomings:
Working Safety:
After driving a round the workmen are in the unprotected area and thus in a particularly hazardous position. On account of the heavy rebound and the generation of dust when bringing the shotcrete in the workmen are exposed to considerable health risks.
Costs:
As the shotcrete is not used completely, because of rebounding, the costs for the material employed in this method are high.
Any possibly required advancing measures of protection add to the costs as these cannot be taken into account for the later supporting capacity of the shotcrete shell.
Personnel:
For implementation, the personnel must be well-trained; it is hard to find such personnel nowadays.
Construction Rate:
As the advancing operation and the shotcrete support work must take place one after another the operations cannot be synchronized. The construction rate is therefore low.
Supporting Capacity:
It is difficult to provide static proof of the individual states of construction. If the life-times are short, the section is driven for plural partial excavations, which increases the settings.
The mechanically driven shield tunnel has the following disadvantages:
As the tunnel machine has to be manufactured individually, in accordance with the respective tunnel geometry and geology, it can in most cases be used only for one order and is therefore subject to high costs. Because of the high installation costs the shield machine is not economical for the construction of short tunnels. Only circular sections can be made. The maximal tunnel section at a given clear section is only in exceptional cases circular, so that there are increased costs because of the additional excavation work. Any variations of the tunnel section in the longitudinal direction of the tunnel (for instance for parking bays in road construction or train stations in underground construction) cannot be made by the machine.
In addition to the shortcomings with the mechanically driven shield tunnel the machine without the shield construction has the following shortcoming:
As the supports are installed with a delay and separately of the driving work, it is difficult to react to variations in the ground condition. If supporting work has to be carried out in the area of the machine the driving work is impeded. Moreover, there is the risk of the machine being buried by pieces of rock, etc.